Cardiac resynchronization therapy (CRT) is a promising and accepted device therapy for patients with systolic heart failure classified in New York Heart Association (NYHA) class III and IV. Indications include patients who, despite optimal medication, are symptomatic, and who demonstrate LV asynchrony. The latter occurs in patients with left bundle branch block (LBBB) and typically presents with a QRS width (measured on an ECG machine) of greater than about 130-150 ms. Herein, “asynchrony” is characterized by a delay in systolic contraction between the intraventricular septum and the left ventricular (LV) free wall.
Currently available CRT bi-ventricular pacing generally employs one lead positioned in operative communication with the right ventricle (RV) and one lead in operative communication with a portion of one of the tributaries of the coronary venous system. The myocardial venous system provides a pathway for deployment of LV stimulation of the lead (and associated electrodes) to operatively communicate with the LV. In most patients, an additional lead is deployed to the right atrium (RA) for atrioventricular (AV) synchronization during pacing. Exceptions for placement of the atrial lead include patients suffering from chronic atrial fibrillation (AF) or having a relatively high AF “burden.” According to such CRT delivery, electrical stimulation of both the RV and LV operates to assist ventricular asynchrony and increase contractility (as measured by ventricular pressure development (dP/dt).
CRT has been established as an effective treatment for heart failure patients (NYHA III, IV) with long QRS duration (QRSd>120 ms) and low ejection fraction (EF<35%). A number of acute studies demonstrated a significant dependence of various indices of cardiac function on the programmed values of the atrio-ventricular (AV) and inter-ventricular (W) delays. The most commonly used methods of AV and VV delay interval optimization are based on echocardiographic evaluation of filling characteristics, cardiac output (CO), and ventricular dyssynchrony for different interval settings. A few chronic studies demonstrated limited evidence of long-term benefit of echo-guided interval optimization. However, considering supposedly incremental benefit of interval optimization such methods seem to be too time and resource-consuming. For certain patients, further optimization of the AV interval can be performed on the guidance of echocardiographic or hemodynamic parameters as is known in the art. However, such methods of optimization of the programmed AV delays in triple chamber (e.g., CRT delivery) implantable medical devices (IMDs) involve complexities. With supposedly incremental benefit of optimization, echocardiographic evaluation simply takes too much time and effort for clinicians (and clinics) and requires coordination between implanting physicians and imaging personnel and equipment. Besides the time, effort and coordination required, the patient is typically lying down and essentially stationary during the procedure. Accordingly, the patient's hemodynamic state during optimization simply does not correlate to the state during activities of daily living (ADL); this is, when the patient is ambulatory.
Thus, there is a need in the art for an improved, easily optimized pacing therapy delivery system that does not need take the above-noted factors into consideration while preserving the benefits of the pacing system described above. Specifically, there is a need for apparatus and methods to easily and efficiently control AV intervals in a rate-adaptive cardiac pacing therapy delivery device (e.g., a dual-chamber or a triple-chamber).